Enable file picker and markdown editor before submitting first prompt of session

### Feature summary

When opening a new session, user can see file picker in right rail and direclty edit, save, and commit files before first session in the prompt

### What problem are you trying to solve?

### TL;DR
As a user curating agent context in my repo, I often want to direclty edit files _before_ I engage an agent. For example, I may want to write a spec or product hypothesis, and then work with an agent to spar on that idea.

### Proposed solution

### Current state:
- Start session
- Right rail not availabel
- Sumbit first prompt of session
- Right rail option appears
- Click right rail, choose file picker, edit files

### Ideal state:
- Start session (local, worktree, etc.)
- RIght rail with file picker appears
- Choose file, edit, and then submit first prompt
- All changes should be able to be saved and commited before first prompt

### Workflow impact

### Why it's worth investing in

- Keeps user in GH app. The absence of a pre-session file picker and editor experience drives users to secondary editors (VS Code, Obsidian, etc.) outside of GH app. Introduces opportuntiy for incomplete session + creates context switching overhead for user.

### Installation context

_No response_

### Additional context

_No response_

Start from the session-start flow and the right-rail visibility condition described in the issue; trace how the file picker, markdown editor, save, and commit actions become available after the first prompt. Done means a new local or worktree session exposes the right rail before the first prompt and lets users choose, edit, save, and commit files.

Rédigé par le modèle d'indexation à partir du texte de l'issue.